Rotationally Selected and Resolved State-to-State Photoelectron Study of Vanadium Monoxide Cation VO+(X3Σ–; v+ = 0–3)

Pulsed-ramped-field-ionisation zero-kinetic-energy photoelectron spectroscopy of the metastable rare-gas atoms Ar, Kr and Xe

The photoionisation of the metastable rare-gas atoms Rg = Ar, Kr and Xe is investigated at the Rg+ […](ns)2(np)5 2P3/2 ← Rg[…](ns)2(np)5((n + 1)s) 13P2 photoionisation threshold (n = 3, 4 and 5 for Ar, Kr and Xe) using the technique of pulsed-ramped-field-ionisation zero-kinetic-energy (PRFI-ZEKE) photoelectron spectroscopy. This technique, which monitors the field ionisation of high Rydberg states induced by a slowly growing electric-field ramp after a prepulse of opposite polarity, was recently introduced to record photoelectron spectra with high resolution and high sensitivity [Harper, Chen, Boyé-Péronne and Gans, Phys. Chem. Chem. Phys., 2022, 117, 9353]. A tunable UV laser system with a bandwidth of 30 MHz is used here to establish the factors determining the resolution and overall accuracy of this new method. In particular, the effects of stray electric fields, and of the magnitude of the prepulse and the field ramp on PRFI-ZEKE photoelectron spectra are studied by combining experiments with numerical simulations of the field-ionisation of high Rydberg states and of the electron flight times from the ionisation spot to the detector. A spectral resolution of 0.05 cm-1 is demonstrated in the photoelectron spectrum of metastable Ar.

Infrared photodissociation spectroscopic investigation on VO+ and NbO+ hydrolysis catalyzed by water molecules

We investigate the hydrolysis of vanadium/niobium monoxide cation (VO⁺/NbO⁺) with water molecules in the gas phase.

Bond dissociation energies of transition metal oxides: CrO, MoO, RuO, and RhO

Through the use of resonant two-photon ionization spectroscopy, sharp predissociation thresholds have been identified in the spectra of CrO, MoO, RuO, and RhO. Similar thresholds have previously been used to measure the bond dissociation energies (BDEs) of many molecules that have a high density of vibronic states at the ground separated atom limit. A high density of states allows precise measurement of the BDE by facilitating prompt dissociation to ground state atoms when the BDE is exceeded. However, the number of states required for prompt predissociation at the thermochemical threshold is not well defined and undoubtedly varies from molecule to molecule. The ground separated atom limit generates 315 states for RuO, 252 states for RhO, and 63 states for CrO and MoO. Although comparatively few states derive from this limit for CrO and MoO, the observation of sharp predissociation thresholds for all four molecules nevertheless allows BDEs to be assigned as 4.863(3) eV (RuO), 4.121(3) eV (RhO), 4.649(5) eV (CrO), and 5.414(19) eV (MoO). Thermochemical cycles are used to derive the enthalpies of formation of the gaseous metal oxides and to obtain IE(RuO) = 8.41(5) eV, IE(RhO) = 8.56(6) eV, D₀(Ru-O^-) = 4.24(2) eV, D₀(Cr-O^-) = 4.409(8) eV, and D₀(Mo-O^-) = 5.243(20) eV. The mechanisms leading to prompt predissociation at threshold in the cases of CrO and MoO are discussed. Also presented is a discussion of the bonding trends for the transition metal oxides, which are compared to the previously measured transition metal sulfides.

The bond dissociation energy of VO measured by resonant three-photon ionization spectroscopy

The predissociation threshold of VO has been measured using resonant three-photon ionization (R3PI) spectroscopy. Given the high density of electronic states in the molecule, it is argued that the molecule dissociates rapidly as soon as the thermochemical bond dissociation energy (BDE) is exceeded, allowing the measured predissociation threshold to be assigned as the BDE. This is the first time a BDE has been measured using the R3PI method. The first photon is provided by an optical parametric oscillator (OPO) laser that promotes VO into a high-energy, discrete vibronic state. A tunable dye laser then excites the molecule further to a resonant state close to the dissociation limit where there is a quasi-continuum of states. A second photon from the same dye laser pulse ionizes the molecule, generating VO⁺ ions. The dye laser is then scanned to higher energies, and when the energy of one OPO photon plus one dye photon exceeds the BDE, the molecule dissociates before another dye photon can be absorbed to induce ionization. The combined photon energy at the sharp drop in the ion signal is assigned as the BDE. The experiment has been repeated using four different intermediate states, all yielding the same BDE, D₀(VO) = 6.545(2) eV. Using thermochemical cycles, a revised value for the BDE of cationic VO is obtained, D₀(V⁺-O) = 6.053(2) eV. The 0 K enthalpy of formation for VO(g) is also derived as Δ_fH_0K ⁰VO(g) = 128.6(1.0) kJ mol^-1. Previous spectroscopic and thermochemical studies of VO are reviewed.

High-Level Ab Initio Predictions for the Ionization Energy, Bond Dissociation Energies, and Heats of Formation of Vanadium Methylidyne Radical and Its Cation (VCH/VCH+)

The ionization energy (IE) of VCH, the 0 K V-CH/VC-H bond dissociation energies (D₀s), and the heats of formation at 0 K (ΔH_f0^°) and 298 K (ΔH_f298^°) for VCH/VCH⁺ are predicted by the wave function-based CCSDTQ/CBS approach. This composite-coupled cluster method includes full quadruple excitations in conjunction with the approximation to the complete basis set (CBS) limit. The contributions of zero-point vibrational energy, core-valence (CV) correlation, spin-orbit coupling, and scalar relativistic corrections are taken into account. The present calculations show that adiabatic IE(VCH) = 6.785 eV and demonstrate excellent agreement with an IE value of 6.774 7 ± 0.000 1 eV measured with two-color laser-pulsed field ionization-photoelectron spectroscopy. The CCSDT and MRCI+Q methods which include CV correlations give the best predictions of harmonic frequencies: ω₂ (ω₂⁺) (bending) = 534 (650) and 564 (641) cm^-1 and the V-CH stretching ω₃ (ω₃⁺) = 835 (827) and 856 (857) cm^-1 compared with the experimental values. In this work, we offer a streamlined CCSDTQ/CBS approach which shows an error limit (≤20 meV) matching with previous benchmarking efforts for reliable IE and D₀ predictions for VCH/VCH⁺. The CCSDTQ/CBS D₀(V⁺-CH) - D₀(V-CH) = -0.012 eV and D₀(VC⁺-H) - D₀(VC-H) = 0.345 eV are in good accord with the experimentally derived values of -0.028 4 ± 0.000 1 and 0.355 9 ± 0.000 1 eV, respectively. The present study has demonstrated that the CCSDTQ/CBS protocol can be readily extended to investigate triatomic molecules containing 3d-metals.

Photodissociation dynamics and the dissociation energy of vanadium monoxide, VO, investigated using velocity map imaging

Velocity map imaging has been employed to study multi-photon fragmentation of vanadium monoxide (VO) via the C 4Σ- state. The fragmentation dynamics are interpreted in terms of dissociation at the three-photon level, with the first photon weakly resonant with transitions to vibrational energy levels of the C 4Σ- state. The dissociation channels accessed are shown to depend strongly on the vibrational level via which excitation takes place. Analysis of the evolution of the kinetic energy release spectrum with photon energy leads to a refined value for the dissociation energy of ground state VO of D0(VO) = 53 126 ± 263 cm-1.

Quantum state control on the chemical reactivity of a transition metal vanadium cation in carbon dioxide activation

By combining a newly developed two-color laser pulsed field ionization-photoion (PFI-PI) source and a double-quadrupole-double-octopole (DQDO) mass spectrometer, we investigated the integral cross sections (σs) of the vanadium cation (V+) toward the activation of CO2 in the center-of-mass kinetic energy (Ecm) range from 0.1 to 10.0 eV. Here, V+ was prepared in single spin-orbit levels of its lowest electronic states, a5DJ (J = 0-4), a5FJ (J = 1-5), and a3FJ (J = 2-4), with well-defined kinetic energies. For both product channels VO+ + CO and VCO+ + O identified, V+(a3F2,3) is found to be greatly more reactive than V+(a5D0,2) and V+(a5F1,2), suggesting that the V+ + CO2 reaction system mainly proceeds via a "weak quintet-to-triplet spin-crossing" mechanism favoring the conservation of total electron spins. In addition, no J-state dependence was observed. The distinctive structures of the quantum electronic state selected integral cross sections observed as a function of Ecm and the electronic state of the V+ ion indicate that the difference in the chemical reactivity of the title reaction originated from the quantum-state instead of energy effects. Furthermore, this work suggests that the selection of the quantum electronic states a3FJ (J = 2-4) of the transition metal V+ ion can greatly enhance the efficiency of CO2 activation.

Bond dissociation energies of TiSi, ZrSi, HfSi, VSi, NbSi, and TaSi

Predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiSi, ZrSi, HfSi, VSi, NbSi, and TaSi. It is argued that because of the high density of electronic states at the ground separated atom limit in these molecules, the predissociation threshold in each case corresponds to the thermochemical bond dissociation energy. The resulting bond dissociation energies are D₀(TiSi) = 2.201(3) eV, D₀(ZrSi) = 2.950(3) eV, D₀(HfSi) = 2.871(3) eV, D₀(VSi) = 2.234(3) eV, D₀(NbSi) = 3.080(3) eV, and D₀(TaSi) = 2.999(3) eV. The enthalpies of formation were also calculated as Δ_f,0KH°(TiSi(g)) = 705(19) kJ mol^-1, Δ_f,0KH°(ZrSi(g)) = 770(12) kJ mol^-1, Δ_f,0KH°(HfSi(g)) = 787(10) kJ mol^-1, Δ_f,0KH°(VSi(g)) = 743(11) kJ mol^-1, Δ_f,0KH°(NbSi(g)) = 879(11) kJ mol^-1, and Δ_f,0KH°(TaSi(g)) = 938(8) kJ mol^-1. Using thermochemical cycles, ionization energies of IE(TiSi) = 6.49(17) eV and IE(VSi) = 6.61(15) eV and bond dissociation energies of the ZrSi^- and NbSi^- anions, D₀(Zr-Si^-) ≤ 3.149(15) eV, D₀(Zr^--Si) ≤ 4.108(20) eV, D₀(Nb-Si^-) ≤ 3.525(31) eV, and D₀(Nb^--Si) ≤ 4.017(39) eV, have also been obtained. Calculations on the possible low-lying electronic states of each species are also reported.